1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising a mechanical position sensor to prevent a runaway condition.
2. Description of the Prior Art
FIG. 1A shows an exploded view of a prior art disk drive comprising a disk 2 rotated by a spindle motor 4, and a head 6 coupled to a distal end of an actuator arm 8 which is rotated about a pivot 10 by a voice coil motor (VCM) in order to actuate the head 6 over the disk 2. The disk 2, spindle motor 4, head 6, actuator arm 8, and VCM are enclosed in a head disk assembly (HDA) comprising a base 9 and a cover 11. The VCM comprises a voice coil 12 coupled to the base of the actuator arm 8 and one or more permanent magnets attached to a yoke 14. When the voice coil 12 is energized with current, the resulting magnetic flux interacts with the magnetic flux of the permanent magnets to generate a torque that rotates the actuator arm 8 about the pivot 10. A tang 16 attached to the actuator arm 8 interacts with a crash stop 18 to limit the stroke of the actuator arm 8, and also provides a latching mechanism (e.g., using a magnet) to maintain the actuator arm 8 in a latched position while the disk drive is powered down. Alternatively, the actuator arm 8 may be parked on a ramp located at the outer periphery of the disk 2 when the disk drive is powered down.
Servo sectors 210-21N are written to the disk 2 to define a plurality of radially spaced, concentric servo tracks 23 as shown in the prior art disk format of FIG. 1B. A plurality of data tracks are defined relative to the servo tracks 23, wherein each data track is partitioned into a plurality of data sectors with embedded servo sectors 210-21N. Each servo sector (e.g., servo sector 214) comprises a preamble 25 for synchronizing gain control and timing recovery, a sync mark 27 for synchronizing to a data field 29 comprising coarse head positioning information such as a track number, and servo bursts 31 which provide fine head positioning information. The coarse head position information is processed to position a head over a target track during a seek operation, and the servo bursts 31 are processed to maintain the head over a centerline of the target track while writing or reading data during a tracking operation. If the servo control circuitry malfunctions during the middle of a seek operation, for example, if a microprocessor malfunctions due to a programming error, or if synchronization to the servo sectors 210-21N is lost, it may cause the actuator arm 8 to collide with the crash stop 18 at a high speed. The resulting vibrations in the actuator arm 8 due to the collision with the crash stop 18 may damage the head 6.
There is, therefore, a need to prevent the actuator arm in a disk drive from colliding with a crash stop at high speeds due to a malfunction in the servo control circuitry.